The present invention relates to a process for preparing o-chloromethylbenzoyl chlorides of the formula I, 
in which R1 to R4 can be identical or different and are hydrogen, C1-C4-alkyl, halogen or trifluoromethyl, by reacting benzo-fused lactones of the formula II, 
in which R1 to R4 are as defined above, with thionyl chloride.
o-Chloromethyl-substituted benzoyl chlorides are important intermediates for preparing, for example, pesticidally active compounds, as described in the patents EP-A 460 575, EP-A 463 488, WO-A 95/18789, WO-A 95/21154 and WO-A 97/15552.
o-Chloromethyl-substituted benzoyl chlorides can be prepared, for example, by reacting benzo-fused lactones with thionyl chloride or phosgene. If thionyl chloride is used, the apparatus to be employed is simplified and safety precautions can be reduced.
EP-A 676 389 describes the preparation of o-chloromethylbenzoyl chlorides from benzo-fused lactones using thionyl chloride in the presence of a nitrogen compound. To achieve a satisfactory conversion, reaction temperatures of 160-170xc2x0 C. are required, at which thionyl chloride is already partially decomposed, resulting in the formation of troublesome byproducts. Furthermore, addition of gaseous hydrochloric acid is required. Finally, in some cases the yields are considerably less than 90%.
In WO-A 99/16743, the reaction with thionyl chloride is carried out in the presence of BF3 etherate and a quaternary ammonium salt at 90-100xc2x0 C. It is found to be disadvantageous here that BF3 etherate is highly sensitive to moisture and may also liberate diethyl ether in the course of the reaction. Diethyl ether has a low boiling point, is readily combustible and tends to form peroxides. Moreover, by hydrolysis of boron trifluoride, it is possible for hydrogen fluoride (hydrofluoric acid), which is highly corrosive and causes problems, in particular with respect to the materials used, to be formed. Implementation on an industrial scale would therefore require special apparatus.
It is an object of the present invention to provide an economical process, suitable for industrial implementation, for preparing o-chloromethylbenzoyl chlorides which does not have the abovementioned disadvantages and still affords high yields.
We have found that this object is achieved by the process mentioned at the outset, which comprises carrying out the reaction in the presence of catalytic amounts of a boron-containing catalyst selected from the group consisting of: boric acid, boric anhydride, borate, boronic acid, boronic acid esters and catalytic amounts of an ammonium salt.
The starting materials used are benzo-fused lactones (phthalides) of the formula II, 
in which R1 to R4 can be identical or different and are hydrogen (H), C1-C4-alkyl, halogen (fluorine, chlorine, bromine or iodine) or trifluoromethyl. Preference is given to using unsubstituted phthalide.
Suitable quaternary ammonium salts are, in particular, quaternary ammonium halides, such as, for example, tetraalkylammonium chlorides or bezyltrialkylammonium chlorides. Particularly suitable are: tetramethylammonium chloride, tetraethylammonium chloride, tetrabutylammonium chloride, benzyltrimethylammonium chloride, benzyltriethylammonium chloride, benzyltributylammonium chloride, N,N-dimethylpiperidininum chloride. Particular preference is given to benzyltriethylammonium chloride.
The quaternary ammonium salts are generally added in amounts of from 0.1 to 20 mol %, based on the amount of benzo-fused lactone used, preferably in amounts of from 0.5 to 10 mol %.
The reaction is furthermore carried out in the presence of catalytic amounts of boric acid, boric anhydride, borate, boronic acid or boronic acid esters. Suitable boronic acids/esters are the following compounds: arylboronic acid (in particular phenylboronic acid), arylboronic acid C1-C4-alkyl esters, C1-C6-alkylboronic acid or C1-C4-alkylboronic acid C1-C4-alkyl esters. suitable borates are in particular alkali metal berates, such as borax.
Particular preference is given to using boric acid. Such processes give excellent yields and have the advantage that the reaction mixtures are free of fluoride ions. Thus, it is not possible for hydrofluoric acid (hydrogen fluoride) to be formed in the course of the reaction. Compared to the analogous reaction where the Lewis acid used is a BF3 derivative, the entire apparatus is simplified.
The boric acid is added in amounts of from 0.1 to 20 mol %, based on the amount of benzo-fused lactone (phthalide) employed, preferably in amounts of from 0.5 to 10 mol %.
Based or the phthalide II in general from 1 to 1.5 equivalents of thionyl chloride are used.
The thionyl chloride can be charged initially together with the other reactants (batch process), or be metered in in the course of the reaction over preferably 1-8 hours (semi-batch process). Furthermore, it is possible to carry out the reaction continuously.
If desired, it is possible to introduce gaseous hydrogen chloride to accelerate ring-opening. However, introduction of hydrogen chloride during the synthesis is preferably dispensed with.
The reaction temperatures used are usually from 100 to 180xc2x0 C. and preferably from 110 to 150xc2x0 C.
The process is preferably carried out in the absence of a solvent. However, it is possible to add a solvent which is inert to thionyl chloride. Inert solvents are, for example, aromatic hydrocarbons, such as toluene, o-, m- or p-xylene or mixtures thereof, chlorinated aromatic hydrocarbons, such as chlorobenzene or dichlorobenzenes, or cyclic carbonates, such as ethylene carbonate or propylene carbonate. It is furthermore possible to use thionyl chloride itself as solvent which can be removed distillatively at the end of the reaction and be recycled into the process.
The reaction is generally carried out at atmospheric pressure or at a pressure of 1-10 bar.